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Driven by quantum information science, quantum photonics has developed significantly in recent years. One example is the use of conventional waveguide technology to facilitate quantum optical processes and algorithms. Alongside this, the field of quantum key distribution (QKD) has matured to the point where commercial products are now available.

Various parameters describe the quantum features and behaviour of quantum optical sources, devices and processes. However, these quantities are not always directly derived from conventional units of the SI system in the same way as quantities such as acceleration, resistance or power. Quantum-optical metrics determine the utility of these sources and devices for quantum photonics applications.

A suite of measurement capabilities has been developed to study a variety of single-photon sources operating at different wavelengths. A confocal microscope (pictured above) captures the emitted photons from the source (figure 1), a Hanbury Brown & Twiss correlator measures g(2)(τ), demonstrating the single-photon nature of the source (figure 2). A Michelson interferometer measures the coherence time τ∞h of the photons; together with the radiative decay time, the transform ratio is obtained. When a source’s photons are Fourier transform limited, the transform ratio 2τrad/τc = 1, and the photons are said to be totally indistinguishable. A consequence of this indistinguishability is two-photon interference; this phenomenon is demonstrated through a Hong-Ou-Mandel experiment , from which the photons’ overlap V(Δt) can be measured. For totally indistinguishable photons, V(Δt=0) = 1.

The focus now is to expand these facilities to characterise quantum photonic components operating at telecom wavelengths for applications such as QKD where the technology will be incorporated into existing telecom networks. NPL are members of the QKD industry standards group within the European Initiative Standards Institute. This group is comprised of industrialists, academics and research scientists that are working towards outlining standards required for QKD.